1. Field
This invention relates to methods and apparatus and systems for time synchronisation in wireless networks and in particular to wireless networks of the type where time synchronisation is important to obtain acceptable performance from receiving units in a wireless network, such as in audio and video functions and other multi-media functions.
2. Related Art
The IEEE 802.11 Wi-Fi standard is commonly used for many wireless applications between computing devices, network servers and the like. It is increasingly being used in domestic applications for functions such as streaming audio data, streaming video data, and other multi-media applications such as gaming using portable gaming devices. Data which is to be transmitted to a plurality of stations (STA) in a wireless network is provided via an access point (AP). This broadcasts the relevant data and it is detected by one or more receivers or stations for which it is intended.
Each station in the network will have circuitry associated with it to process the data in accordance with the type of data received. For example, a pair of wireless speakers will each receive left and right channels of an audio signal and will have to decode this and play it back for a user to listen to. The circuitry in each station will have a free running clock. This is usually a crystal based oscillator and it is common for these not to be particularly accurate, and they are therefore often not synchronised with each other between stations. Thus, for e.g. a pair of loud speakers the free running clock used to control playback may be faster in one loud speaker than in the other which will gradually cause the left and right channels to become less synchronised.
Time protocols such as NTP, e.g. NTPRFC1305, are commonly used to provide a strict timing mechanism over standard wired distribution systems e.g. Internet wired networks.